Polymeric nanostructures are quite distinct because of the large ratio of surface-to-volume macromolecules which result in very different physical and mechanical behavior compared to bulk. Of special interest are nanostructures in which the constituent polymer is in its glassy state to provide structural and dimensional stability against surface forces that are particularly strong at the micron and submicron scales. The majority of existing literature on the mechanical properties of ultra-small volumes of polymers in their glassy state focuses on ultra-thin films and is limited to small deformations and the viscoelastic regime. Experiments with composite materials encapsulating polystyrene (PS) thin films and in their glassy state at room temperature have demonstrated shear yielding and large deformations that are not possible at the macroscale where PS fails at small strains due to crazing. This dissertation research focused on direct experiments with atactic PS nanofibers to elucidate and quantify the unusual viscoplastic response of ultra-small volumes of PS as a function of the underlying molecular and structural length scales. PS is an ideal polymer for this study because it is amorphous and its glass transition temperature, Tg, is much higher (100C) than room temperature.
The objective of this research was to understand the synergistic coupling between the material length scale as defined by macromolecular size, and the specimen size as defined by the fiber diameter, which can result in extreme ductility and simultaneous strengthening and toughening for fiber diameters at the submicron scale. To this goal, PS fibers with diameters 150–5,000 nm were electrospun from monodisperse PS powders with molecular weights, MW, in the range 13,000–9,000,000 g/mol. Individual nanofibers were tested using a surface micromachined device for nanofiber testing at the quasi-static strain rate of 10-2 s-1. Unlike the brittle behavior of bulk PS, the engineering stress vs. stretch ratio of individual nanofibers with several combinations of MW and diameter displayed very repeatable post-yield behavior including necking and pronounced strain-hardening. Specifically, the ratio of the structural length scale (fiber diameter) to the intrinsic macromolecular length scale (root-mean-square end-to-end chain distance), Dnorm, was shown be an excellent scaling parameter to determine the occurrence and evolution of necking and strain hardening in submicron scale PS fibers. This interplay between molecular and structural length scales in glassy PS fibers was favorably exploited to harness a ~3,000% increase in toughness along with simultaneous increase in tensile strength: the highest fiber strength was achieved for Dnorm = 3–5, whereas increasing Dnorm resulted in gradual decline in strength. Bulk-like brittle behavior took place for Dnorm > 18. It was shown that the effects of molecular and structural lengths scales on large deformation behavior of fibers could be collapsed onto a single master curve as long as the MW was larger than the critical value for constant inter-chain entanglement length. Furthermore, it was shown that the pronounced hardening in PS nanofibers is not a result of post yield necking, but part of the material constitutive response: experiments on individual poly(lactide-co-glycolic acid) (PLGA) nanofibers showed that unlike in bulk specimens, nanoscale imperfections and specimen irregularities are rather benign and strong post-yield strain hardening occurs even when necking is suppressed.
The viscous component of the large deformation response in PS nanofibers was assessed by tensile experiments with PS nanofibers with MW = 123,000 – 2,000,0000 g/mol and diameters of 200 – 750 nm in the range of strain rates 10-4 - 102 s-1. For fibers with Dnorm < 8, it was shown that increasing strain rate resulted in monotonic increase of the stress amplitude without affecting the large fiber stretch ratios. In contrast, the strain rate influenced both the stress and the stretch ratio of fibers with Dnorm > 10, i.e. fibers without significant post-neck hardening. For all PS fibers, the rate dependent stress vs. stretch ratio curves scaled with yield stress. Therefore, a normalized stress vs. strain curve could be generated to combine size effects and temporal effects on the mechanical behavior of PS nanofibers at room temperature.